1 / 47

MEIOSIS

MEIOSIS. Meiosis. The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n) Meiosis is sexual reproduction. Two divisions ( meiosis I and meiosis II ). sperm n=23. n=23. egg. 2n=46 zygote. Fertilization.

woolley
Download Presentation

MEIOSIS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. MEIOSIS

  2. Meiosis • The form of cell divisionby which gametes, with half the number of chromosomes,are produced. • Diploid (2n)  haploid (n) • Meiosis is sexual reproduction. • Two divisions (meiosis I and meiosis II).

  3. sperm n=23 n=23 egg 2n=46 zygote Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg

  4. Meiosis • Sex cells divide to produce gametes(sperm or egg). • Gametes have half the # of chromosomes. • Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis • Meiosis is similar to mitosis with some chromosomal differences.

  5. n=23 human sex cell sperm n=23 n=23 2n=46 haploid (n) n=23 n=23 diploid (2n) n=23 meiosis I meiosis II Spermatogenesis

  6. Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

  7. Interphase I • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. • Centriole pairs also replicate.

  8. Interphase I • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

  9. Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. • four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

  10. Prophase I • Longest and most complex phase (90%). • Chromosomes condense. • Synapsis occurs:homologous chromosomes come together to form a tetrad. • Tetrad is two chromosomes or four chromatids(sister and nonsister chromatids).

  11. Homologous chromosomes sister chromatids sister chromatids Tetrad Prophase I - Synapsis

  12. Homologous Chromosomes • Pair of chromosomes (maternal and paternal) that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each locus(position of a gene) is in the same position on homologues. • Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes b. 01 pair of sex chromosomes

  13. Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type.

  14. eye color locus eye color locus hair color locus hair color locus Paternal Maternal Homologous Chromosomes

  15. Humans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues. Homologue Homologue

  16. Autosomes(The Autosomes code for most of the offspring’s traits) In Humans the “Autosomes” are sets 1 - 22

  17. 21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female?

  18. Sex ChromosomesThe Sex Chromosomes code for the sex of the offspring.** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male

  19. Y chromosome X chromosome Boy or Girl? The Y Chromosome “Decides”

  20. Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata. • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. • Chiasmata (chiasma) are the sites of crossing over.

  21. Tetrad nonsister chromatids chiasmata: site of crossing over Crossing Over - variation variation

  22. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

  23. Sex Chromosomes XX chromosome - female XY chromosome - male

  24. spindle fiber centrioles aster fibers Prophase I

  25. Metaphase I • Shortest phase • Tetrads align on the metaphase plate. • INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

  26. OR metaphase plate metaphase plate Metaphase I

  27. Question: • In terms of Independent Assortment -how many different combinations of sperm could a humanmale produce?

  28. Answer • Formula: 2n • Human chromosomes: 2n = 46 n = 23 • 223 = ~8 million combinations

  29. Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres.

  30. Anaphase I

  31. Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed.

  32. Telophase I

  33. Meiosis II • No interphase II (or very short - no more DNA replication) • Remember: Meiosis II is similar to mitosis

  34. Prophase II • same as prophase in mitosis

  35. metaphase plate metaphase plate Metaphase II • same as metaphase in mitosis

  36. Anaphase II • same as anaphase in mitosis • sister chromatids separate

  37. Telophase II • Same as telophase in mitosis. • Nuclei form. • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg

  38. Telophase II

  39. n=2 sex cell sperm n=2 n=2 2n=4 haploid (n) n=2 n=2 diploid (2n) n=2 meiosis I meiosis II Meiosis

  40. Variation • Important to population as the raw material for natural selection. • Question: What are the three sexual sources of genetic variation?

  41. Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!

  42. Question: • A cell containing 20 chromosomes(diploid)at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  43. Answer: • 10 chromosomes (haploid)

  44. sperm n=23 n=23 egg 2n=46 zygote Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg

  45. Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  46. Answer: • 10 chromosomes

  47. Sources • www.ursulinehs.org/powerpoint/meiosis.ppt • www.biology4teachers.com/Cell%20Division/MEIOSIS.ppt • http://www.iteachbio.com/Life%20Science/LifeFunctionsandTheCell/Meiosis.ppt

More Related